This application relates to a process for removing contaminated sediments from the bottom of lakes, reservoirs, rivers, streams, and other water bodies, while at the same time minimizing the moisture content present in these extracted sediments.
Current methods of dredging can be divided into two general categories. They include mechanical dredging and hydraulic dredging. The fundamental difference between these categories is in the form in which the sediments are removed. Mechanical dredges remove the sediments directly with clamshell-type buckets. The operation consists of lowering the bucket with a crane to the bottom of the waterway, scooping or extracting the sediment, and bringing the sediment to the surface for disposal (typically in a dredge barge). Hydraulic dredges, sometimes referred to as vacuum dredges, are designed to vacuum up bottom sediments. Unless the sediments are very loose, vacuum dredges require cutter heads or alternative means to dislodge the dredge material so that the sediment can be vacuumed into the dredge head. Mechanical dredging operations typically yield much lower liquid to solid ratios (30 to 70 percent by weight) compared to hydraulic dredges (less than 1 to 10 percent by weight).
Due to the very high cost of contaminated sediment removal and disposal, which is typically priced on the basis of $ per unit volume or $ per unit weight (e.g., $ per cubic yard or $ per ton), the presence of moisture in the sediment will unavoidably raise the price of a dredging cleanup. The higher the moisture content, the higher the cost of disposal.
As previously noted, mechanical dredges tend to produce less moisture per weight of mud (lower liquids to solids ratio), but the sediment is exposed to the water column during the excavation process, which tends to supersaturate the excavated sediment. Due to the very high moisture content associated with hydraulic or vacuum dredging operations, the muds collected from such operations typically employ costly dewatering processes to try to reduce the quantity of moisture in the mud, thereby reducing the overall disposal cost.
The invention being disclosed herein also relates to the development of a system equipped to collect sediment from the marine environment with minimal moisture content, thereby reducing the cost of disposal over that of conventional dredging methods.
It is therefore an object of this invention to provide for the removal of sediments from the subsurface of a water body with low moisture, closely approximating the in-situ moisture content of the mud sample.
It is further an object of the present invention to improve over the disadvantages of the prior art.
In this embodiment, the system provides for housing a mechanical bucket in a specially designed enclosure into which positive pressure, equivalent to the hydrostatic pressure present at the corresponding submergence depth, is introduced into the enclosure to prevent water from entering the enclosure and providing the means to submerge, excavate and retrieve the sediment in a dry environment. As a result, such sediments can be recovered at a moisture content that approximates that of the in-situ condition of the sediment.
The operation of this system includes the following steps:
a) enclosing a dredge bucket within a sealed housing having an openable door;
b) providing a source of positive air pressure to said housing;
c) maintaining said positive air pressure at an amount equal to a predetermined hydrostatic pressure imposed by a surrounding water column surrounding said housing, thereby preventing the introduction of water from said surrounding water column into said housing;
d) submersing said housing having said bucket down to the in situ contaminated sediments to be dredged;
f) extending said bucket into said in situ contaminated sediments and collecting the contaminated sediments having said predetermined low moisture content into said excavator bucket;
g) retracting said bucket under positive pressure within said housing;
h) closing said openable door attached to said housing;
i) lifting of said positively pressurized housing with said bucket with to the surface with said dredged contaminated sediments having said low moisture content; and,
j) disposing of said contaminated sediments with said low moisture content to a disposal site.